(1) Field
The disclosed systems and methods relate generally to ion beam scanning of workpieces and in particular to semiconductor wafers. Specifically, the systems and methods are directed to linearly scanning multiple wafers during a single pass of an ion beam scanning so that the ion beam is more efficiently utilized and the throughput of the wafers is increased.
(2) Description of Relevant Art
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity is ionized in an ion source, the ions ale accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a (as or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. Various ion implanting systems are known. In one such conventional ion implanter, an ion beam is used that is much smaller than the wafer in both the vertical and horizontal directions. This implanter distributes the implant dose across the wafer by scanning the beam, moving the wafer or a combination of beam scanning and wafer movement. However, in these systems, the ion beam is not directed onto the wafer for the entire scan time and the efficiency of the beam scan suffers. It is desired to maximize the efficiency of the ion beam by directing the beam onto the wafer or the largest amount of scan time as possible.
In another type of known ion implanter, a fixed width ion beam is utilized that is greater than the diameter of the wafer and is only scanned vertically. In contrast, the ion beam is scanned both horizontally and vertically if a small sized spot beam is used for implantations. In one example of a fixed width ion beam, a ribbon ion beam is selected so that the width of the ion beam is slightly larger than the diameter of the wafer and the beam is vertically scanned across the wafer. Presently, wafer diameter sizes of 200 mm and 300 mm and flat panel displays exceeding 300 mm are commonly used with larger sizes expected. In these cases, the fixed beam width must be chosen to be greater than the largest expected workpiece size.
However, a great inefficiency of the beam results when a smaller workpiece than the maximized design width of the ion beam size is used. For instance, a fixed beam width of 300 mm may be used to meet the present standard for 300 mm wafers as shown in FIG. 2(a) and a fixed beam width of 200 mm is used for systems where the maximum workpiece size is 200 mm or less. The beam width wb of the 300 mm beam is designed to be slightly larger than the 300 mm wafer, for instance a fixed beam width of approximately 330 mm may be used with a scan distance ds of approximately 380 mm. Similarly, the beam width wb of the 200 mm beam is designed to be approximately 230 mm and has a scan distance ds of approximately 280 mm. However, when a 200 mm (or smaller) wafer is implanted with a fixed beam width of 300 mm, the beam is inefficiently used as shown in FIG. 2(c). Whenever a wafer is used that is smaller than the designed size for the ion beam, an inefficiency in use of the beam will result.
In another type of a conventional ion implanter, a variable width beam is utilized to approximate the shape of the wafer. Width sensors are used to detect the width of the wafer and the speed of the wafer movement is controlled to compensate for dose variations which result from different sweep widths. Also, it is known to match the width of the target wafer with the width of the scan beam by varying the sequence of scan times. Such systems require complex detection and processing devices. Furthermore, variable width beam systems may not be used in many systems due to limitations and constraints on the electronics and architecture of such implanting systems.
Accordingly, an ion implanter that increases the efficiency of the fixed width beam while scanning workpieces for applications such as ion implanting.